1. Field of the Invention
This invention relates generally to digital data modems and more specifically to such modems which have a capability for rapidly adapting to the characteristics of a communication channel.
2. Description of the Prior Art
Multipoint communication networks typically include a master station which selectively communicates with one of a plurality of slave stations. The number of slave stations varies with a particular installation between five and twenty stations, for example. A transmitter and a receiver in the master station are coupled by telephone lines to a transmitter and a receiver at each of the slave stations.
In communication networks of this type, the transmitter of the master station and the receivers of the slave stations are on-line at all times. The master station polls, in turn, each of the slave stations to ascertain if there is any data to be transmitted. If a particular slave station has data to send to the master station, it will respond to a "polling" signal by sending an "acknowledge" signal, followed by the data to be transmitted.
Before the receiver in the master station can accurately receive data from one of the slave stations, it must adapt its characteristics to the characteristics of the telephone lines. These characteristics of the telephone lines will vary with each of the slave stations due primarily to the variations in the distances of the respective slave stations from the master station. Once the receiver has adapted to the telephone lines, the relatively slow methods of the prior art may be sufficient to track the changing line characteristics. However, when a receiver first goes "on-line" its characteristics may differ significantly from those of the telephone lines. Under these "start-up" conditions, the relatively slow tracking methods of the prior art may take as long as two seconds to adapt the receiver to the line. This period of adaptation, which is multiplied by the number of slave stations for a single polling of the network, has significantly reduced the data throughput of the prior art systems.
Various attempts have been made to provide special start-up procedures to facilitate the fast adaptation of the receiver to the telephone lines. Typically, a special data sequence is transmitted by the transmitter to provide a signal with known characteristics in the receiver. In response to this special sequence, timing and phase error signals have been produced. These error signals have been highly dependent upon the accuracy of an automatic gain control in the receivers. As a consequence, there has been a high probability of incorrect timing and phase learning.
Although these adaptation techniques of the prior art have been somewhat effective, it is always desirable to reduce the adaptation time of a receiver. Any such reduction in time is multiplied by the number of stations in the multipoint communication network to provide a significant reduction in the adaptation time associated with a single polling of the network. It is further desirable to provide phase timing and equalization update signals which are independent of signal magnitude.